Hidden voice recorders are designed to capture audio without detection. They work by recording audio in a discreet manner, usually in the form of a small device that can be hidden or disguised. These recorders may have features like voice activation or noise reduction to ensure high-quality audio capture. They may also have built-in storage or the ability to store audio on a separate device like an SD card. Hidden audio recorders can be powered by batteries or USB cables, depending on the device. Some recorders also have wireless capabilities and can be accessed remotely through a smartphone app or computer. Overall, these devices work by providing a covert means of audio recording for a variety of applications.